The treatment of aqueous waste streams with a significant content of organic matter has been and continues to be a matter of substantial technological interest. These waste streams may be general sewage or they may be from various industrial operations such as food processing or paper production.
A very old but still valid approach to this problem is to reduce the dissolved and colloidally suspended organic content of the waste stream through microbiological action. This typically involves digestion of this organic matter by aerobic fermentation, anaerobic fermentation or both. Such microbiological activity generates a gelatinous material commonly referred to as activated sludge. It has a high tightly bound water content.
The disposal of such sludges is also a matter of technological concern. The two primary goals have been to terminate the biological activity and to reduce the water content. Both have been addressed by either the treatment of the sludge with lime (calcium hydroxide) or the heat treatment of the sludge with or without the addition of an oxygen bearing gas or an oxidizing agent. Neither approach has been entirely satisfactory.
The lime treatment is expensive and actually significantly increases the amount of sludge with which one must deal. A fairly high loading of lime, typically between about 150 and 550 pounds per ton of dry sludge solids, is required for dewatering enhancement. The expense of such a treatment involves not only the costs of the lime itself but also the substantial labor costs in handling such a large quantity of additive. Furthermore, effective dewatering also requires the addition of an inorganic metal salt, typically ferric chloride, at loadings commonly between about 40 and 125 pounds per ton of dry sludge solids. Most of the lime and all of the ferric chloride usually end up as additional solids in the filter cake thus adding to the amount of solid material of which one must dispose.
In addition, some sludges require even higher loadings of lime for stabilization. An EPA case study has reported a need for as much as 1000 pounds per ton of dry sludge solids to reliably terminate biological activity.
On the other hand, heat treatment has apparently uniformly involved the heating of rather dilute feed streams with typical solids contents between about 3 and 10 weight percent. This heat treatment has almost universally involved the use of some type of heat exchanger to recover some of the heat energy expended in raising the temperature of the very high water content feed stream. Scaling problems are commonly encountered in passing these dilute sludges through such heat exchangers resulting in a frequent need to acid wash the heat exchanger.
Furthermore, two separate operations are usually required to dewater the heat treated sludge. Generally, the heat treated material is fed to a decanter, clarifier or settling tank and then the underflow from this operation is fed to a rotary vacuum filter or a recessed chamber pressure filter.
There are disclosures of feeding a fairly dilute heat treated material directly to a centrifuge for final dewatering. However, such procedures require the addition of a substantial quantity of flocculating polymer.